1. Field of the Invention
This invention relates to a process for producing an ethylenic polymer. Specifically, the invention relates to a process for producing an ethylenic polymer by polymerizing ethylene or copolymerizing ethylene with another .alpha.-olefin using a novel catalyst system having superior polymerization activity. More specifically, the invention relates to an improved process for producing an ethylenic polymer which comprises polymerizing ethylene or copolymerizing ethylene with another olefin by using a catalyst system obtained from a solid catalyst ingredient prepared by contacting a tetravalent titanium compound with a pre-treated carrier and an organoaluminum compound, which catalyst system has superior polymerization activity and can afford a copolymer of ethylene and another .alpha.-olefin in which the distribution of short-chain branches with regard to its molecular weight (the distribution of the degree of branching) is relatively uniform.
2. Description of the Prior Art
In recent years, a number of suggestions have been made about improved catalysts for polymerization of ethylene with or without other .alpha.-olefins which comprise transition metal halides (especially halogen compounds of titanium) supported on a carrier (especially a compound of a divalent metal, especially magnesium) optionally pre-treated with a certain specific compound. These catalysts have been alleged to have a markedly increased polymerization activity per unit weight of catalyst (catalytic activity) and to permit substantially omission of the step of removing the catalyst residue from the resulting polymers after the end of polymerization.
Examples of the carrier used in these catalysts are magnesium compounds such as magnesium hydroxychloride, magnesium oxide, magnesium alcoholates, magnesium hydroxide and magnesium dihalides. Products obtained by pre-treating these magnesium compounds with such compounds as water, alcohols, aldehydes, ketones, esters, ethers or carboxylic acids have also been suggested.
These carriers or their pre-treated products have led to a marked increase in polymerization activity, but none have proved to be entirely satisfactory in the polymerization of ethylene with or without other olefins. Among the difficulties associated with these carriers are:
(1) The catalysts do not have sufficiently feasible polymerization activity. Although they have high activity for a relatively short period of time, they will be heavily deactivated with time. Hence, their productivities in long-term polymerization are low.
(2) The bulk density of the resulting polymer is too low. Hence, the productivity of the polymerization apparatus decreases.
(3) With these catalysts, the effect of a molecular weight regulator (generally, hydrogen) used to control the melt flow index of the resulting polymer is relatively small. Hence, polymers having a high melt index cannot be obtained unless the molecular weight regulator is used in a relatively large amount (in the case of using hydrogen, its partial pressure is elevated), or the polymerization is performed at high temperatures.
(4) With these catalysts, the reactivity of the other olefin as a comonomer in copolymerization with ethylene is poor, and therefore, high concentration of the comonomer are required. In other words, since the conversion of the comonomer is lower than that of ethylene, the comonomers is wasted.
(5) In copolymerization of ethylene with other olefins using these catalysts, the uniformity of the distribution of the branching degree of the copolymer is insufficient (branching is considerably distributed in low-molecular-weight copolymers, but it is scarcely distributed in high-molecular-weight copolymers). Hence, the copolymers cannot exhibit sufficient performance.
In the high-density polyethylene industry, copolymers of ethylene with other olefins are widely produced, and most of the molding materials for blow molding, stretching, film formation, etc. are the copolymers.
The basic properties of these copolymers are greatly determined by the amount and distribution of the comonomers introduced into the ethylene chain. In particular, the number of short-chain branches present in the high-molecular-weight portion greatly affects the basic properties of the polymers such as the crystalline structure, the degree of crystallization, and the rate of crystallization, and also many practical properties such as resistance to environmental stress cracking, film strength, softness, and moldability.
For these reasons, it is very important from the viewpoint of improving practical properties to control the number and distribution of short-chain branches in an ethylene copolymer.
The distribution of the degree of branching in the copolymer seems to be greatly affected by such factors as the copolymerization reactivity with the comonomer and the ease of chain transfer by a molecular weight regulator (especially, hydrogen). In copolymers of ethylene and olefins obtained by conventional catalyst systems such as an organoaluminum compound (for example, trialkyl aluminums)-titanium trichloride system and an organoaluminum compound-titanium tetrachloride system, short-chain branches are present mainly in the low-molecular-weight portion. Hence, they reduce the density of the copolymer and increases its hydrocarbon-soluble portion, and cannot greatly change the basic properties of the polymer such as its melting point and the rate of crystallization.
The reactivity of the comonomer in copolymerization also appears to have closely to do with the distribution of the degree of branching. Particularly, in the case of copolymerization of ethylene with an olefin containing at least 4 carbon atoms, the conversion of the comonomer is a major factor of the cost of production.
When the performances of prior art catalysts for copolymerization of ethylene with olefins which are based on a halogen compound of titanium supported on a carrier obtained by pre-treating the magnesium compounds with the electron donor are examined, it is found that the uniformity of the distribution of the degree of branching and the reactivity of the comonomer are considerably improved as compared with unsupported transition metal compound catalysts, but are not entirely satisfactory. In particular, these prior art catalyst systems decrease drastically in polymerization activity with time. Hence, all of these properties should be greatly improved.
The problem in commercial application of catalyst systems having a high initial polymerization activity but a tendency to be deactivated greatly with time is that even if the average polymerization time is prolonged, the productivity (the output per unit amount of catalyst) does not increase, and moreover, this causes great troubles to the operation of the process, such as the risk of radicality of the polymerization reaction and the difficulty of controlling the reaction temperature.
Many methods involving the use of magnesium alcoholates as the aforesaid carrier have been suggested. They include, for example, the following.
(1) Method in which a reaction product formed between a magnesium alcoholates and a tetravalent titanium compound is used as a solid catalyst ingredient.
(2) Method in which a magnesium alcoholates is contacted with an alkyl aluminum halide and then the product is reacted with a transition metal halide.
(3) Method in which a titanium tetraalkoxide is reacted with a magnesium alcoholates and the reaction product is reacted with silicon tetrachloride.
(4) Method in which a magnesium alcoholates is reacted with a halogenating agent, and then a transition metal compound is supported on the product.
When ethylene is polymerized, or copolymerized with another olefin, in the presence of catalyst systems obtained from the transition metal compounds supported on a carrier by these methods and an organometal compound (especially, an organoaluminum compound), the aforesaid problems cannot be completely solved, and therefore, these catalysts are neither satisfactory for practical purposes.
Thus, in spite of the fact that ethylenic polymers, above all copolymers of ethylene with olefins, have attained an increasingly important status in commercial production because of their broad range of utility, there has been scarcely any suggestion about catalyst systems which are suitable for producing ethylenic polymers having improved basic properties.